At a rally in Iowa this week, Donald J. Trump explained in bold terms and broad brush strokes the many ways he would improve the United States as president: building a wall, rebuilding the military, beating China on trade. When a protester chimed in to question how the Republican would do the things that he promised, Mr. Trump decided that instead of removing the man from the audience, he would respond. "You watch," the billionaire developer said flatly. His supporters cheered. Mr. Trump has regained momentum in Iowa two weeks before the state's caucuses, and many voters cite his bravado...